1. Field of the Invention
The present invention relates to a chip resistor and a method of making a chip resistor.
2. Description of the Related Art
FIG. 16 shows a conventional method of making a chip resistor (see JP-A-2002-57009). In this method, first, as shown in (a) in the figure, two metal plates 90′ and 91′ are prepared. Then, as shown in (b), the metal plate 91′ is bonded to the lower surface of the metal plate 90′. Then, as shown in (c), part of the metal plate 91′ is cut away by machining to form a gap 93. Then, as shown in (d), a solder layer 92′ is formed on the lower surface of the metal plate 91′. Then, the metal plates 90′ and 91′ are cut as shown in (e), whereby a chip resistor B is obtained. The chip resistor B manufactured in this way includes a pair of electrodes 91 on the lower surface of the resistor element 90 which are spaced from each other via the gap 93. To enhance the solderability in the mounting process, a solder layer 92 is formed on the lower surface of each of the electrodes 91.
Generally, in incorporating a chip resistor in a circuit to make a product, it is preferable that part of solder adheres to an end surface of the resistor element of the chip resistor to form a solder fillet. The bonding portion at which a solder fillet is formed has a high bonding strength, and the electrical conduction at the bonding portion is reliable. Thus, the bonding condition can be easily determined by checking the presence or absence of a solder fillet from the outside. That is, when the presence of a solder fillet is observed from the outside, it can be determined that the chip resistor is properly mounted. Conversely, when the presence of a solder fillet is not observed from the outside, it can be determined that the chip resistor is not properly mounted.
In the above-described chip resistor B, e.g. Ni—Cu-based alloy, Cu—Mn-based alloy or Ni—Cr-based alloy is used as the material of the resistor element 90. However, these alloys do not have a sufficient solder wettability, so that it is difficult to form a proper solder fillet directly on the two end surfaces of the resistor element 90. Although the solder layer 92 is provided on the lower surface of each electrode 91, the provision of the solder layer 92 is not sufficient for forming a solder fillet. When the chip resistor B is to be surface-mounted on an object by reflow soldering, cream solder is applied in advance to a portion of the object to which the chip resistor B is to be mounted. However, when the amount of the cream solder is insufficient, a proper solder fillet is not formed. In this way, a solder fillet is not reliably formed in the conventional method.
Further, the conventional method of making the chip resistor B requires the steps of bonding the metal plates 90′ and 91′ together and machining the metal plate 91′. These are troublesome work which takes relatively a long time, so that the making efficiency of the chip resistor B is deteriorated.